The present invention relates to a planar optical waveguide and a method for manufacturing the same.
Communication systems utilising optical systems having become common place recently. When first introduced, the optical systems were based on the use of optical fibers which were symmetrically round. More recently however, planar waveguide devices have been introduced due to the ease with which different types of device can be formed utilising techniques learned from semiconductor manufacture. However, planar waveguides frequently exhibit differing refractive indices parallel and perpendicular to the plane of the surface of the substrate. Unpolarised light, which enters a birefringent planar waveguide is split into one component in a direction parallel to and one component in a direction perpendicular to the surface of the substrate of the waveguide and these components propagate at different rates. This makes optical circuits more difficult to design because wavelength--selective elements such as directional couplers or wavelength multiplexers incorporating reflection gratings can be optimally designed for only one polarization direction. This phenomenon is termed by birefringence and in the case of crystalline planar substrates, the birefringence results from the crystalline structure and the selected orientation of the crystals to the plane. Amorphous, transparent materials such as glass do not exhibit birefringence provided they are stress free.
Much attention has therefore been directed to producing low-birefringence planar optical waveguides and the technique normally employed is to manufacture the waveguides from glass on a essentially planar silicon crystal substrate. With this method, the layers of glass are produced at elevated temperature and/or require high temperature treatment in order to ensure homogeneity. The result of the high temperature treatment is that the difference between the thermal expansion co-efficients of the substrate and the layers coated thereon therein leads to considerable stresses on cooling and, in the event of single-sided coating, even to bending of the silicon crystal substrate. Owing to the well known stress-optical effect, these stresses bring about birefringence in the light carrying core of the waveguide. The effect of the birefringence is shown in FIG. 3 where the peak insertion losses in all TE and TM modes occur at different wavelengths.
A number of different proposals have been made to overcome this problem and while some techniques are capable of achieving very low polarisation sensitivity (less than or equal to 0.05 nm for arrayed waveguide demultiplexers) they are not suitable for low cost/high volume production. DE-A-4433738 discloses a technique which is stated to result in low-birefringence and involves making the thermal coefficient of expansion of the optical core material the same as the temperature coefficient of expansion of the silicon substrate. This document is alleged to result in a reduction in the polarisation sensitivity in the region of 0.1 to 0.2 run which is still not sufficiently good for practical purposes.